1. Field of the Invention
The present invention relates to cannulas used in surgical applications, and more particularly, to a cannula equipped with a pressure/flow rate transducer.
2. Description of Related Art
In medical applications and specifically in surgery, the list of uses for cannulas is exhaustive. One application involves the augmenting or supplementing of pulmonary blood flow through the beating heart during cardiosurgery by use of one or more cannulas involved in the intake and return of blood into the circulatory system. The cannulas interface between the patient's circulatory system and the mechanical pumps that power the augmentation procedure. Such an application is described in co-pending PCT Application no. PCT/US97/18674 entitled "Single Port Cardiac Support Apparatus", filed Oct. 14, 1997 and incorporated herein by reference in its entirety.
When performing cardiac surgery cannulas are placed within the patient's blood stream and used for inflow and outflow of blood or other fluids. If the operator wishes to determine the rate of fluid flow, either a catheter with appropriate sensors must also be placed in the patient's blood stream, or other sensors such as an external ultrasonic sensor as disclosed in U.S. Pat. No. 5,179,862 are used. A shortcoming of ultrasonic systems such as that described in U.S. Pat. No. 5,179,862 is that they require significant monitoring. Ultrasonic sensors also require that tubing of a specific diameter be used, thereby adding to the cost and complexity of the surgical procedure. Additionally, ultrasonic sensors are expensive and non-disposable, thereby adding to the cost of the surgical procedure.
Another method to measure flow rate is through the use of a thermodilution catheter. Thermodilution catheters require the infusion of a solution, typically saline, of a known temperature, with a distally disposed thermistor measuring the temperature change to determine the flow rate. This method is also expensive, increasing the cost of the surgical procedure. A second problem with using flow-sensing catheters, such as thermodilution catheters, is that they require the operator to place more incisions within the patient. The catheters must be placed so that they do not interfere with the inflow or out flow of the cannula. Visual markers along the length of the cannula may also be used to determine location, the greater the number of markers the more accurate the placement at the expense of quick readings due to the greater number of markings.